1. Field of the Invention
The present invention relates to a tracking control method and apparatus for use with an optical disc system which is suitable for high-density recording of information.
2. Description of the Prior Art
Some conventional optical discs have recording tracks along a predetermined spirally continuous guide groove. Usually, information is written in the recording tracks in the form of grooves, and lands between the track grooves are left blank and used to read the written information without error. On certain optical discs, there is employed sampled format for each track in order to determine positions where the recording tracks are to be formed.
When information is recorded on or reproduced from an optical disc through an optical pickup, the optical pickup is subject to a tracking control process in order to enable a light beam to follow the recording tracks on the optical disc automatically.
For recording or reproducing information on or from an optical disc of the continuous groove type, for example, as shown in FIG. 1 of the accompanying drawings, a light beam reflected from an optical disc D passes through an objective lens 2 and a beam splitter 3 of an optical pickup 1 and reaches a photodetector 4, which produces a tracking control signal. The tracking control signal is then supplied to a two-axis actuator 5 to actuate the optical pickup 1 radially with respect to the optical disc D.
In FIG. 1, the photodetector 4 comprises a two-segment photodetector which is composed of two light-detecting elements 4l, 4r that are symmetric with respect to the center of a track. A tracking error is detected as the difference between the detected output signals from the respective light-detecting elements 4l, 4r. This tracking control method is known as a push-pull method.
According to the push-pull method, as shown in FIG. 2, the diameter of a beam spot SPo on the optical disc D is substantially the same as a track pitch p of 1.6 .mu.m, for example.
If it is assumed that the beam spot is spaced a distance x from the center of any track Ti in the radiation direction of the optical disc D, then detected output signals Sl, Sr produced by the light-detecting elements 4l, 4r, respectively, from the reflection of the beam spot SPo have spatial periods equal to the track pitch p, are spatial phase shifted 1/2 from the phase of the track pitch p, and are represented by sine waves that are opposite in phase, as shown in FIGS. 3A and 3B. The detected output signals Sl, Sr are indicated by the following equations (1a), 1(b): EQU Sl=sin(2.pi.x/p) (1a) EQU Sr=-sin(2.pi.x/p) (1b).
As shown in FIG. 3C, the difference Sso between the detected output signals Sl, Sr is represented by a sine wave which crosses the zero value in the positive direction at the center of the track Ti. The difference Sso is given by the following equation (2): EQU SSo=2 sin (2.pi.x/p) (2)
A three-spot method as shown in FIG. 4 may be used to detect a tracking error on an optical disc (see, for example, U.S. Pat. No. 3,876,842).
According to the three-spot method, two auxiliary beams of diffracted light of the first order are applied to an optical disc in addition to a recording or reproducing light beam (main beam), such that two auxiliary beam spots SPa, SPb are formed on the optical disc by the applied auxiliary beams in point symmetry with respect to a main beam spot SPo formed by the main beam.
The diameter of each of the beam spots is substantially the same as a track pitch p of 1.6 .mu.m, for example. The auxiliary beam spots SPa, SPb and the main beam spot SPo are spaced apart radially of the optical disc by 1/4 of the track pitch p.
Two auxiliary beams reflected by the optical disc from the auxiliary beam spots are detected by respective light-detecting elements of an optical pickup.
If it is assumed that each beam spot is spaced a distance x from the center of any track Ti in the radiation direction of the optical disc, then detected output signals Sa, Sb produced by the light-detecting elements, respectively, from the reflection of the auxiliary beam spots SPa, SPb are represented by sine waves that are opposite in phase, also as shown in FIGS. 3A and 3B. The detected output signals Sa, Sb are indicated by the following equations (3a), 3(b): EQU Sa=sin(2.pi.x/p) (3a) EQU Sb=-sin(2.pi.x/p) (3b).
Also as shown in FIG. 3C, the difference Sab between the detected output signals Sa, Sb is represented by a sine wave which crosses the zero value in the positive direction at the center of the track Ti. The difference Sab is given by the following equation (4): EQU Sab=2 sin(2.pi.x/p) (4)
The differential signal, i.e., the signal indicative of the difference Sab, is used as a tracking control signal in the three-spot method.
One type of optical disc employs a sampled format in which wobbling pits are provided for the detection of a tracking error, for example. On such an optical disc, as shown in FIG. 5, a pair of wobbling pits (servo pits) PTa, PTb is formed in a particular region (servo region), the wobbling pits PTa, PTb being spaced apart from each other along a track and spaced from the center of the track by a distance which is 1/4 of the track pitch p. A reference control pit PTo is also provided on the center of the track in a position spaced certain distances from the servo pits PTa, PTb along the track.
When information is recorded on or reproduced from the optical disc, the servo pits PTa, PTb are scanned by a light beam having a diameter which is approximately the same as the track pitch p. Detected output signals Sa, Sb which are produced in timed relation to the respective servo pits PTa, PTb are represented by sine waves that are opposite in phase, and expressed by the following equations (5a), (5b): EQU Sa=-sin(2.pi.x/p) (5a) EQU Sb=sin(2.pi.x/p) (5b).
Also as shown in FIG. 3C, the difference Sba between the detected output signals Sa, Sb is represented by a sine wave which crosses the zero value in the positive direction at the center of the track Ti. The difference Sba is given by the following equation (6): EQU Sba=2 sin(2.pi.x/p) (6).
The differential signal, i.e., the signal indicative of the difference Sba, is used as a tracking control signal on the sampled-format optical disc.
In the case where the track pitch p is reduced to the extent that the reciprocal of the track pitch p exceeds the spatial cutoff frequency of the optical pickup, the optical pickup cannot read the recorded information and produce an optical image, thus failing to generate a tracking signal.
For example, if the wavelength .lambda. of light and the numerical aperture NA of the objective lens are EQU .lambda.=0.7 .mu.m, NA=0.5,
then the spatial cutoff frequency fc is given as follows: EQU fc=2NA/.lambda.=1/.lambda..apprxeq.1280/mm.
In the range of p.ltoreq..lambda.=0.78 .mu.m, i.e., when the tracking pitch p is smaller than the wavelength .lambda. of light, it is unable to effect any tracking servo process.